strange vibration

[crazyhorse357]

Hi Guys,
I have an old 93 G-20 chevy van with a 4.3 six and an 4L60E and it has fairly recently developed a strange vibration, now a little history, about 6 months ago I took it to tranny shop because it was shifting hard between 1st and 2nd. The guys there told me that they could take care of it by drilling a hole in one of the valves in the valve body, well I let them do it and absolutely nothing changed, shortly after that this vibaration appeared.
I took it back to the tranny guys and they said there was nothing wrong with the tranny, converter was locking and unlocking and it was shifting fine, with the exception of the hard 1st to 2nd shift, and they pointed out that the vibration was there when it was not moving, and it is, at a certain rpm it can be felt.
They then told me to have the motor mounts/tranny mounts checked and I did, nothing wrong there, mechanic also checked the harmonic balancer and he said it's ok.
I am running out of places to look, I gave the serpentine belt a good look and it's ok.
Anyone got anymore ideas? cause I'm out.
crazyhorse357
I forgot to say that the vibration is most present when the tranny shifts into 3rd and also in O/D

[lonestar1128]

checked the spark plug boots for carbon? it would be a little black line going down the inside of the boot and grounding thru the engine rather than the spark plug. would cause a noticeable erratic vibration at low rpm (~1000-2500 usually), significant loss of power... thats all i can think of tho

[crazyhorse357]

Thanks LoneStar,
I don't know exactly what you're refering to, Do you mean the spark plug wire itself or is it a different wire, don't know of another wire going to the spark plugs ????
crazyhorse

[lonestar1128]

after re-reading your post i think i was too tired when i responded.. lol.. if it's at specific rpm range, almost seems like something is unbalanced in the rotating assembly? what rpm does the vibration seem the worst? and does it occur with the trans in park/neutral?

i think my spark plug wire theory is way off now, but this pic resembles what a carbon trail would look like at the end of the wire boot.

[jimbo543]

u-joint maybe

[crazyhorse357]

I would have thought a u-joint as well, however the vibration is present when parked and tranny in park as well, don't have a tachometer, but just guessing I would say vib. begins somewhere in the 2500-3000 rpm range and that being the range I would guess the engine would be in at about 60 mph and the tranny in O/D.

[75K30]

If your vibration is present when the truck/van is not in motion, then the engine has got to be causing the vibration. WHile it is entirely possible that the trans could be causing the problem, this is highly unlikely.
You mentioned that the harmonic balancer was checked, but I I was a betting man, I would bet that the balancer has slipped, and is no longer in the correct orientation. Now we know that it cant slip on the crank, although I have seen them crack around the index key, but again doubt the problem is here. I would carefully inspect the rubber band that divides the outer and the inner portion of the balancer/damper. These are two piece, and after the rubber dries out, can allow the outer portion to slip, and become out of balance.
Here is a little something to keep in mind while inspecting: it may be very difficult to visually inspect the balancer, since it is possible for this to move without leaving it painfully obvious to tell.
Since the timing marks are located on the outside of the balancer, you can often determine a problem by using a plain old fashioned timing light. Having a good one that is adjustable will be important. You already know what the factory specifications for timing will be, so setting the dial on the timing light to this specification at this time, and looking for the timing marks to read "zero" would verify a problem with the balancer. Since the timing light is adjustable, the dial actaully has a delay based upon the adjusted value. This prevents you from having to use a long timing tape that usually falls of anyway, and you can use your stock timing marks.
Hope this makes sense, sometimes understanding how newer timing lights can be a bit of a mystery, but this method should tell you if you are off a few degrees.
While it is possible that the engine timing is off a few degrees anyway, but this method should tell you if you are looking in the right direction.
My vote is still with the balancer/damper. Have seen them fail a few too many times.
Good luck.

[436'd skylark]

Originally Posted by 75K30

If your vibration is present when the truck/van is not in motion, then the engine has got to be causing the vibration. WHile it is entirely possible that the trans could be causing the problem, this is highly unlikely.
You mentioned that the harmonic balancer was checked, but I I was a betting man, I would bet that the balancer has slipped, and is no longer in the correct orientation. Now we know that it cant slip on the crank, although I have seen them crack around the index key, but again doubt the problem is here. I would carefully inspect the rubber band that divides the outer and the inner portion of the balancer/damper. These are two piece, and after the rubber dries out, can allow the outer portion to slip, and become out of balance.
Here is a little something to keep in mind while inspecting: it may be very difficult to visually inspect the balancer, since it is possible for this to move without leaving it painfully obvious to tell.
Since the timing marks are located on the outside of the balancer, you can often determine a problem by using a plain old fashioned timing light. Having a good one that is adjustable will be important. You already know what the factory specifications for timing will be, so setting the dial on the timing light to this specification at this time, and looking for the timing marks to read "zero" would verify a problem with the balancer. Since the timing light is adjustable, the dial actaully has a delay based upon the adjusted value. This prevents you from having to use a long timing tape that usually falls of anyway, and you can use your stock timing marks.
Hope this makes sense, sometimes understanding how newer timing lights can be a bit of a mystery, but this method should tell you if you are off a few degrees.
While it is possible that the engine timing is off a few degrees anyway, but this method should tell you if you are looking in the right direction.
My vote is still with the balancer/damper. Have seen them fail a few too many times.
Good luck.

good info, but I'd take a strong look at the other end of the motor as well. I've replaced countless flexplates on 4.3s in heavy applications such as yours. All sorts of cracking until it comes apart.

also, 4.3s crankshafts are a split pin design. while suitable for light duty work, but under sever stress they tend to crack, which starts as a vibration, then turns into a ticking, then it splits in two and total engine failure occurs. pretty nasty stuff, usually wastes the entire short block.

I've also seen excessively worn clutch fans cause alot of problems too.


the 4.3s are decent motors, but they really have no business in such a large vehicle.

[75K30]

Good call once again Joe!